Brake specs

[brianinpa]

I am looking for some help with a problem that I am having with the rear brakes on my 81. The master cylinder is the original 81 which was for a single piston caliper. The caliper I am using is a dual piston caliper from an 84. I have rebuilt everything and installed a SS brake line and I am 90% sure I have the brake completely bled (I am buying a vacuum bleeder later this week), but I can only build up enough brake pressure by pumping the pedal a few times. I am beginning to think maybe the MC isn't pumping enough fluid to move both pistons at once. Was there a difference in fluid output of MCs?

 

[dan filipi]

Not sure on your question but I do know the earlier mc doesn't push as much volume as an 83 mc that pushes the one front and the rear. I tried it before modding in an 1800 mc.

 

[slabghost]

81 likely does not push as much fluid as 84 is my guess. However once the pads are seated it should be sufficient. Might want to try pushing the fluid back with a C clamp on the caliper which may allow any remaining air to bubble out the master. Be sure to hold the caliper below the brake line. 83 and 84 rear masters are for both the front and rear.

 

[joedrum]

ok hooch has 1000 master and 1200 brake with no problems ....i would back bleed the system using pistons to push fluid back in master rear goes fast as it is short ...just need clamps that fit the pistons

 

[brianinpa]

I've done all of that which is why I am pretty sure all the air is out of the system, but you never really know, so I amgettng a vacuum bleeder to make sure.

 

[joedrum]

sheesh it just came to me that a 1000 master operates a 2 piston brake set up that the pistons are much bigger than 1200 pistons are by at least double....

 

[canuckxxxx]

On my '82 the rear brake was mushy even after I had bled it thoroughly. So I took the rear caliper off and set it in a low position (on a coffee can), put a piece of 1/4" plate where the disc would go, and blocked the brake pedal down with a piece of wood. Left it that way over night and there was a noticeable improvement the next day.

Something to try.

Brian

 

[wedoo2]

I too rebuilt both ends of my 1100 and felt the mushiness, even the brake by 2 X 4 feel, but after riding it a day or two they both now feel great. I replaced the front MC with a Yamaha part that was just a couple of years old. Stainless, rebuilt calipers, new banjo bolts, pads, whole nine yards. Maybe it just needs to settle in.

 

[jpwinger]

Mite check brake pedal to make sure it is giving full travel to master Jerry

 

[brianinpa]

I have tried everything I know to do to these rear brakes, but I just cannot get any brake pressure without pumping the pedal 4 or 5 times. I really think it is a matter of the 1100 MC not putting out enough fluid to move the dual Pistons.

Looks like I have some more modifying to do, but the more I read, the more I thin I am going with an 1800 MC.

 

[slabghost]

Good idea. Might take a bit of modification to fit it.

 

[mcgovern61]

FYI Brian, I am using the 81 rear master on my '82 and it is pushing the dual caliper easily. I do not know if the '84 caliper is that much bigger, but it might be. My problem with that is that it is hydraulic. Once the fluid level is up and the pistons make contact with the pads and the pads contact the rotor, be sure the reservoir is full. If so, then I am more inclined to believe there is air in there.

 

[brianinpa]

If there is air still in there, it sure is hiding well. I have used a vacuum bleeder and also back bled them... Still nothing.

 

[backlander]

Do you have linked brakes ? Never mind, answered my own question. My only thought is possibly the bore or piston is worn out of spec and there is enough bypass around the piston for it to not allow suffienct buildup of pressure. If they had of been linked you might have had a problem with the proportioning valve not working properly.

 

[joedrum]

hmmmmm wonder what going on with brakes lately ....baffling me ...these parts should work fine together

 

[brianinpa]

Nope... Not linked brakes. I have tried two different calipers also. One that I rebuilt and then I ok the one off of my 84 1200 rear that I knew was working... Still no brakes which is why I think it is the amount of fluid coming from the MC.

 

[backlander]

I edited my post and you might not of seen where I ask about the possibility of out of spec wear on the mc bore and/ or piston. Did you rebuild the m/c?

 

[dan filipi]

This sounds a lot like when I tried an aftermarket mc that was too small for my linked brakes. Had to pump it up quick many times to get any pedal.

 

[Tom_Charlton]

For what it is worth, the 81 rear master cylinder has a different part number than the 83 rear master cylinder. I would suspect the 81 does not push enough fluid to work the 83 dual slave cylinder rear brake. Can you try using an 84 rear master cylinder, which displaces enough fluid for the linked front dual slave cylinders and the rear dual slave cylinders?

 

[mcgovern61]

I am still at basic hydraulics here. While I agree you might not have enough to get a good push on the rotors with the smaller MC, but it should still fill the lines and pistons with fluid and take good pressure. Pumping up only proves air in the system.